1. Field of the Art
The present invention relates to a multiple-time ink-bearing medium containing a thermally transferable ink material, which is used for a thermal printer.
2. Related Art Statement
In the field of thermal printing, there has been used a thermal print ribbon made of a sheet (hereinafter referred to as "thermal print sheet") which comprises a substrate, and a layer of a thermally-fusible and -transferable ink composition formed on one surface of the substrate. The ink composition consists of a coloring agent and a binder. In use, the ink composition on the heated portions of the print ribbon is transferred to a recording sheet of paper. Therefore, the thermal print ribbon, once used, can not serve again, namely, the same area of the print ribbon cannot be used two or more times. Accordingly, such a "one-time" or "single-use" thermal print ribbon is not economical to use.
To overcome the above economical drawback, the following three different types of thermal print sheets for multiple-time thermal print ribbons have recently been proposed:
The first proposed thermal print sheet is disclosed in Japanese Patent Application which was laid open in 1982 under Publication No. 57-160691. This print sheet for a multiple-time thermal print ribbon is prepared by mixing carbon black or other coherent or coagulant powder into an ink composition which consists of a fusible dye and a material having a low melting point, and by applying the mixture to a substrate. The carbon black or similar coherent powder contained in such a thermal print sheet does not at all contributes to formation of an image, that is, the image is formed by the fusible dye.
Since images to be formed by dyes tend to be easily faded, the above thermal print sheet suffers from a problem of permanency of a printed image, i.e., does not provide a long life of printed documents.
The second proposed thermal print sheet for a thermal print ribbon comprises an ink-impregnated layer of heat-resistant resin formed on a substrate, which resin layer has a large number of continuous small pores or voids filled with a fusible ink. This thermal print sheet or ribbon is disclosed in Japanese Patent Application laid open in 1980 under Publication No. 55-105579. However, this type of thermal print sheet requires complicated steps for forming the porous resin layer on the substrate, and is difficult to efficiently impregnate the porous resin layer with the fusible ink with uniform distribution.
Accordingly, the second proposal suffers from low uniformity of optical density of printed images, and therefore fails to provide satisfactory printing quality.
The thermal print sheet of the third proposal is prepared by using a solution of a resin in which an ink material is dissolved and/or dispersed. This solution is applied to a substrate, and the solvent in the coating is evaporated. As a result, a finely porous layer of the resin is obtained, which contains masses of fusible ink. This method is disclosed in Japanese Patent Application laid open in 1979 under Publication No. 54-68253.
However, any of the thermal print sheets disclosed in the documet indicated just above requires an extremely larger amount of energy input to fuse the ink material, than the conventional thermal print sheet for a one-time thermal print ribbon.